The present invention relates to a multi-stage DCxe2x80x94DC converter wherein a plurality of converters is connected in series, and particularly to a multi-stage DCxe2x80x94DC converter wherein control of the output voltage is performed by means of negative feedback of the output voltage.
In recent years, DCxe2x80x94DC converters used for communication have lower output voltages and higher currents and, accordingly, improving efficiency is even more important than previously. To this end, techniques of increasing efficiency by connecting two converters in series have attracted attention. Such DCxe2x80x94DC converters are called two-stage DCxe2x80x94DC converters.
Known methods of controlling two-stage DCxe2x80x94DC converters include a method of achieving stability of operation by optimizing the values of the inductors and capacitors within the converters and also the equivalent series resistance values of capacitors (see P. Alou et al.; xe2x80x9cBuck+Half Bridge (d=50%) Topology Applied to Very Low Voltage Power Converters,xe2x80x9d IEEE Applied Power Electronics Conference (APEC) 2001).
In this method, specifically, the resonance frequencies of the inductors and capacitors of the first stage and second stage are separated from each other and the values of the equivalent series circuits are enlarged to achieve stable operation of the control system.
However, when a two-stage DCxe2x80x94DC converter is manufactured as an actual product, because of limitations with respect to the shape and mounting surface area, it is difficult to use inductors and capacitors that are optimal for control. In addition, because of standards for the output ripple voltage, it is difficult to increase the equivalent series resistance value of capacitors.
In addition, with a single-stage DCxe2x80x94DC converter, there are known methods of achieving stable operation of the control system using multiple loops, but with two-stage DCxe2x80x94DC converters, there are many unknown areas with respect to what kind of multiple-loop scheme can be used to achieve stability of operation of the control system. This point is the same for DCxe2x80x94DC converters wherein three or more converters are connected in series.
It is therefore an object of the present invention to provide a multi-stage DCxe2x80x94DC converter that uses multiple loops in the control system and that can achieve stable operation.
In addition, another object of the present invention is to provide a multi-stage DCxe2x80x94DC converter with a simple construction that achieves stability by means of a multiple-loop control system even in cases wherein stabilization of the control system is difficult with single-loop output voltage feedback control.
In order to solve the aforementioned problem, a multi-stage DCxe2x80x94DC converter was analyzed by means of state-space averaging, and state equations were derived so that stable control can be performed based on a multiple loops.
Specifically, a multi-stage DCxe2x80x94DC converter as one aspect of the present invention is a multi-stage DCxe2x80x94DC converter comprising: a non-isolated DCxe2x80x94DC converter that is able to adjust the intermediate voltage by PWM control as the first stage, an isolated DCxe2x80x94DC converter as the second stage and a PWM controller that controls the duty cycle of switching of the first stage by means of negative feedback of the output voltage of the second-stage converter, characterized in that the intermediate voltage of the first-stage converter is provided as negative feedback to the PWM controller.
Here, in a preferred embodiment of the present invention, the intermediate voltage is provided as negative feedback to the PWM controller via an operational amplifier.
With this configuration, the input voltage is converted to pulses by the switching of the PWM controller in the first-stage converter and also, the intermediate voltage is adjusted based on the duty cycle of switching, and moreover voltage conversion is performed at the transformer winding ratio in the second-stage converter, thus generating an averaged output voltage.
In this case, not only is the output voltage of the second-stage converter provided as negative feedback to the PWM controller via an external loop, but the intermediate voltage of the first-stage converter is also provided as negative feedback via an internal loop.
Thereby, compared with the case of an external loop alone, the resonance frequency on the low-frequency side in the transfer function of the entire control system is shifted to a higher frequency and accordingly, the frequency at which the phase crosses zero degrees also becomes higher. Accordingly, the gain margin is increased and the operation of the control system is stabilized.
In addition, a multi-stage DCxe2x80x94DC converter as another aspect of the present invention is a two-stage DCxe2x80x94DC converter comprising: a non-isolated DCxe2x80x94DC converter that is able to adjust the intermediate voltage by PWM control as the first stage, an isolated DCxe2x80x94DC converter as the second stage and a PWM controller that controls the duty cycle of switching of the first stage by means of negative feedback of the output voltage of the second-stage converter, characterized in that the intermediate voltage of the first-stage converter is provided as positive feedback to the PWM controller.
With this configuration, the input voltage is converted to pulses by the switching of the PWM controller in the first-stage converter and also, the intermediate voltage is adjusted based on the duty cycle of switching, and moreover voltage conversion is performed at the transformer winding ratio in the second-stage converter, thus generating an averaged output voltage.
In this case, not only is the output voltage of the second-stage converter provided as negative feedback to the PWM controller via an external loop, but the intermediate voltage of the first-stage converter is also provided as positive feedback via an internal loop.
Thereby, compared with the case of an external loop alone, the resonance peak on the low-frequency side in the transfer function of the entire control system vanishes and accordingly, the frequency at which the phase crosses zero degrees also becomes much higher. Accordingly, the gain margin is increased considerably and the operation of the control system is even more stabilized.
In addition, a multi-stage DCxe2x80x94DC converter as still another aspect of the present invention is a two-stage DCxe2x80x94DC converter comprising: a non-isolated DCxe2x80x94DC converter that is able to adjust the intermediate voltage by PWM control as the first stage, an isolated DCxe2x80x94DC converter as the second stage and a PWM controller that controls the duty cycle of switching of the first stage by means of negative feedback of the output voltage of the second-stage converter, characterized in that the switching current of the first-stage converter is provided as feedback to the PWM controller.
With this configuration, the input voltage is converted to pulses by the switching of the PWM controller in the first-stage converter and also, the intermediate voltage is adjusted based on the duty cycle of switching, and moreover voltage conversion is performed at the transformer winding ratio in the second-stage converter, thus generating an averaged output voltage.
In this case, not only is the output voltage of the second-stage converter provided as negative feedback to the PVVM controller via an external loop, but the switching current of the first-stage converter is also provided instead of a reference signal via an internal loop.
Thereby, compared with the case of an external loop alone, the resonance frequency on the low-frequency side in the transfer function of the entire control system vanishes and becomes an inflection point and accordingly, fourth order delay becomes third order delay. Accordingly, the gain margin is further increased and the operation of the control system is even more stabilized.
In addition, a multi-stage DCxe2x80x94DC converter as still another aspect of the present invention is a two-stage DCxe2x80x94DC converter comprising: a non-isolated DCxe2x80x94DC converter that is able to adjust the intermediate voltage by PWM control as the first stage, an isolated DCxe2x80x94DC converter as the second stage and a PWM controller that controls the duty cycle of switching of the first stage by means of negative feedback of the output voltage of the second-stage converter, characterized in that the current flowing through the second-stage converter is provided as feedback to the PWM controller.
With this configuration, the input voltage is converted to pulses by the switching of the PWM controller in the first-stage converter and also, the intermediate voltage is adjusted based on the duty cycle of switching, and moreover voltage conversion is performed at the transformer winding ratio in the second-stage converter, thus generating an averaged output voltage.
In this case, not only is the output voltage of the second-stage converter provided as negative feedback to the PWM controller via an external loop, but the current flowing through the second-stage converter, e.g. the output inductor current, is also provided instead of a reference signal via an internal loop.
Thereby, compared with the case of an external loop alone, the phase in the transfer function of the entire control system no longer crosses zero degrees. Accordingly, the operation of the control system is constantly stabilized.
Note that in the present invention, xe2x80x9cmulti-stagexe2x80x9d refers to where two or more converters are connected in series.